(1) Field of the Invention
The present invention generally relates to liquid crystal display devices. This invention more specifically relates to novel technology for reflective liquid crystal displays.
(2) Background Information
The demand for liquid crystal displays (LCDs) has increased substantially in recent years with the proliferation of computer technology. For example, the use of LCDs in personal computers, video cameras, televisions, cellular telephones, watches and a host of other electronic devices is common. The widespread use and acceptance of LCDs has led to increased demands for continued innovation in this technology, to produce, for example, hand held, more energy efficient, and less expensive display devices.
The development of the reflective liquid crystal display is one attempt to meet these demands. A reflective LCD relies on ambient light to display images. Substantially all reflective LCDs, therefore, have a reflector to reflect the incident light back towards the viewer. In a conventional reflective LCD an aluminum mirror has typically been utilized. One difficulty associated with an aluminum mirror is that it is a specular reflector. A specular reflection is generally undesirable because it includes a reflected image of the light source and other objects (including the user), which may distract the viewer from focusing on the displayed image. As a result, practical reflective LCDs typically diffuse the reflected light enough to blur parasitic reflective images, but not so much as to blur the displayed image. In conventional reflective LCDs, diffused reflection is achieved one of two ways; either by a adding a diffuser film, typically at the front of the display stack, or by patterning the inner surface of the glass substrate upon which a metal mirror is deposited. Conventional reflective displays are generally rendered colored by a separate color filter layer that uses light absorption in each pixel to provide, for example, red, green and blue colors. This display structure, wherein a separate component layer is required for each function: color rendering, reflection, and light diffusion, results in a relatively large number of components and tends to result in a more complex production process and higher costs.
Further, the 90xc2x0 twisted nematic (TN) liquid crystal configuration, which is the dominant technology for transmissive displays, generally cannot be used in conventional reflective LCDs. As a result, other LC configurations tend to be used to achieve adequate brightness modulation at each pixel. Most of these other configurations are generally inferior to the 90xc2x0 TN in one or more aspects. For example, these LC configurations tend to suffer from a subset of the following deficiencies: an insufficiently dark black state, low brightness, low contrast, individual primary colors having distinct modulation curves, an unbalanced white state (without electric field), relatively high voltage operation, requirement for tight tolerances on cell gap and/or a limited field of view. Any of one or more of these limitations may degrade the LCD performance and/or raise the cost of fabrication. One further potential disadvantage of the non-90xc2x0 TN configurations is that they are not able to take advantage of the manufacturing infrastructure already in place for transmissive LCDs.
In one aspect, the present invention includes a reflective liquid crystal display (LCD) including a cholesteric liquid crystal polarizing device, a liquid crystal cell, and an internal quarter-wave retarder. The cholesteric liquid crystal polarizing device, the liquid crystal cell, and the quarter wave retarder are superposed with one another. In variations of this aspect, the reflective LCD may be a normally white mode or normally black mode device. In another variation of this aspect, the liquid crystal cell may include a 90xc2x0 twisted nematic liquid crystal.
In another aspect, this invention includes a reflective LCD including a linear polarizer, the linear polarizer having a polarization direction, a liquid crystal cell, a quarter-wave retarder, the quarter-wave retarder having a fast axis, a cholesteric liquid crystal polarizing device including a plurality of pixel regions, and an absorbing medium.
In a further aspect, this invention includes a method for fabricating a reflective LCD, the method including providing a liquid crystal cell disposed adjacent to a thin film transistor array having a plurality of pixel regions and superposing the liquid crystal cell with a cholesteric liquid crystal polarizing device.
In still another aspect this invention includes a cholesteric liquid crystal polarizing device. The device includes a substrate, an alignment layer, and a cholesteric liquid crystal polarizing layer including a plurality of pixel regions. The pixel regions are arranged in a repeating array of red pixels, green pixels and blue pixels, the red pixels reflecting circularly polarized red light, the green pixels reflecting circularly polarized green light and the blue pixels reflecting circularly polarized blue light.